The invention is concerned with acridinium compounds which can be used as chemiluminogenic labels.
Chemiluminogenic acridinium compounds having formula 2 wherein substituent R.sup.4 contains a functional group capable of reacting with biologically interesting compounds are known from European Patent Applications 82,636 and 216,553. Labelling of nucleotides with such acridinium compounds is known from EP-A-212,951.
Chemiluminescence is the phenomenon that electromagnetic radiation (light) is emitted as a consequence of a chemical reaction. Usually, a reaction product is involved which, because of the nature of the reaction, is produced in an electronically excited state. This excited state product can revert to the "normal" ground state by losing the excess energy in the form of electromagnetic radiation. With acridinium compounds like the ones described in the European Patent Application 82,636 (see also Clin. Chem. 29 (8), 1474-1479 (1983)), chemiluminescence occurs by reaction with basic hydrogen peroxide in a way as shown schematically for compound 2 in FIG. 1.
Chemiluminogenic compounds, just like compounds containing a radioactive isotope, can be used as labels to detect substances which are labelled with these compounds. The advantages of chemiluminogenic labels over radioactive ones are the smaller health hazard, the higher sensitivity, and the longer shelf life.
The use of this kind of labels is particularly relevant in biological systems. In this way not only proteins, carbohydrates, nucleic acids, and other biologically relevant compounds can be determined, but it is also possible to monitor the biological reactions of these compounds with suitable reaction partners. Examples include the interaction of a drug with its receptor, of a toxin with its receptor or antidote, and, more specifically the immunological antigen-antibody reaction.
A special assay system which uses the chemiluminescence reaction is a system wherein the energy of the excited state product (the donor) is not directly emitted as light, but is transferred radiationless to a suitable acceptor, e.g. a fluorogenic compound. This acceptor is capable of losing this energy as electro-magnetic radiation with a wavelength differing from the wavelength of the donor chemiluminescence. This phenomenon has been described by e.g. T. Forster (Ann. Phys. (Leipzig) 2, 55-75 (1948) and Z. Naturforsch. 4a, 321-334 (1949)). Because the wavelength of the emitted light is specific for the acceptor or the donor-acceptor complex, the acceptor which might be an antibody labelled with a fluorogenic compound or a donor-complexed substrate (e.g. an antigen) can be determined without the necessity of separating the bound complex. This so-called homogeneous chemiluminescence energy transfer assay has been described using isoluminol derivatives as the label, see A. Patel and A. K. Campbell, Clin. Chem. 29 (9), 1604-1608 (1983). However, a drawback of luminol and isoluminol and derivatives is their relatively low quantum yield; moreover, a catalyst is needed to start the chemiluminescence reaction of these compounds.
A disadvantage of the acridinium compounds described in the European Patent Applications 82,836 and 216,553) is that they are not useful to perform homogeneous immunoassays based on energy transfer chemiluminescence. Another disadvantage of those prior art compounds is their limited stability in biological fluids.
Now, a class of chemiluminogenic compounds has been found which are useful as labels both for heterogeneous and homogeneous biological assays like immunoassays, and which have an enhanced stability.
The acridinium compounds according to the invention are characterized by the formula 1, wherein
A is a divalent organic moiety,
X is a substituent which can form a dioxetane together with he C-9 acridine atom and hydrogen peroxide,
Y is a counter ion, and
Z is a functional group,
and wherein the benzene rings may carry one or more substituents, such as lower alkyl, lower alkoxy, halogen, etc.
On the one hand, the compounds according to the invention have a chemiluminogenic function, i.e. they can be transformed by a chemical reaction into an electronically excited state product which loses its excess of energy in the form of light. This function is present at C-9 of the acridine. On the other hand, they have a coupling function, i.e. they possess via an organic spacer a functional group capable of binding with e.g. biologically active substances like proteins, but also with e.g. toxicologically relevant compounds. The coupling function is situated on the acridine nitrogen atom. By this twofold function the compounds are useful as label, e.g. for immunoassay purposes. By a specific coupling with the analyte or with a reagent to determine the analyte the presence of the coupling product or of the interaction with the reagent can be detected using chemiluminescence.
The advantage of the acridinium compounds according to the invention is the fact that during the chemiluminescence reaction i.e. the reaction with hydrogen peroxide to give an excited acridone via a dioxetane, the coupling of emitter and analyte is maintained. As a result the energy of the excited state product can be transferred to an acceptor e.g. fluorescein and Lucifer Yellow which is present elsewhere in the coupling product or the complex. In this case the energy transfer is more efficient than with chemiluminescence donors of the isoluminol type, and furthermore, no catalyst is required to start the chemiluminescence.
A further advantage of the compounds according to the invention is the fact that the cause of the chemiluminescence reaction of the coupling product (a compound e.g. a biologically active compound like a steroid labelled with the acridinium compound) is dependent on the presence of a compound which reacts with the labelled compound e.g. an antibody or antidote. With immune reactions of this sort the kinetics of the chemiluminescence of the immune complex differs from the kinetics of the chemiluminescence of the non-complexed labelled compound (For a related case see European Patent Application 103,469). This may be determined as a change in time of maximal emission of chemiluminescence. According to this mechanism there is no need to separate the immune complexes resulting also in a homogeneous immunoassay.
The divalent organic moiety A functions as a so-called spacer, bridging the distance between acridinium moiety and functional group of the substrate. This substrate is bound by way of group Z. The length of A is such that reaction of functional group Z with a functional group of the substrate is possible. This minimal length is therefore dependent upon the substrate which has to be coupled with the compound represented by formula 1. The nature of group A is not important as long as this group does not interfere with the reactivity of functional group Z or with the desired reaction of group X with hydrogen peroxide. The moiety A may contain alkylene groups, arylene groups, carbonyl groups, heteroatoms like nitrogen, oxygen and sulfur etc., e.g. combined in the form of ester groups, thioester groups, amide groups etc. The group A may also be built from bifunctional units e.g. dicarboxylic acids, dithiols, hydroxy-acids, aminoacids, thioacids etc.
Although group A may be any non-interfering divalent organic moiety, in the preferred embodiment of the invention to avoid complicated synthetic steps a simple organic residue is chosen like a divalent hydrocarbon moiety which may contain one or more hetero-atoms and/or carbonyl groups. More specifically, A is an alkylene moiety with 1-4 carbon atoms.
X may be any group capable of forming a 1,2-dioxetane after reaction with hydrogen peroxide. In addition to the two oxygen atoms originating from hydrogen peroxide, this dioxetane contains also the C-9 atom of the acridine. This is why group X has to be a carbon atom substituted with a group capable of leaving e.g. as a negatively charged ion upon reaction with hydrogen peroxide. Examples of substitution groups include a halogen atom, an alkoxy, aryloxy, sulfonamido or alkylthio group, or an ammonium or sulfonium group. Examples of group X include 1-haloalkyl, 1-alkoxyalkyl, cyano, etc. Preferentially, group X is a alkoxycarbonyl or an activated carbamoyl group or more specifically an aryloxycarbonyl group or an arylsulfonylcarbamoyl group.
The anion Y can be any anion like halogen, sulfate, arenesulfonate, etc. As an example Y may be the ion which acts as the leaving group during the quaternization reaction at the acridine nitrogen atom.
The group Z is a functional group capable of coupling with an organic substrate. The choice of Z depends on the functional groups present in the substrate. If the substrate contains e.g. amino groups, hydroxy groups, or mercapto groups, then Z may be a carboxyl group or a reactive derivative thereof, e.g. an ester, acid chloride, etc. Coupling is then achieved by formation of a (thio)ester or (thio)amide bond. A suitable group which is also used in European Patent Application 82,636 is the N-succinimidyloxycarbonyl group. Z may also be isocyanate or isothiocyanate; coupling is then achieved in the form of a urea or urethane, or of a mono- or dithio analogue thereof. Other possible Z groups capable of reacting with amino, hydroxy, and/or mercapto groups include halides, optionally activated at the .beta.-position by a carbonyl or alkene functional group, derivatives of sulfonic acids and phosphoric acids, azides, and optionally protonated carboximidates.
If the substrate contains carboxyl groups or derivatives thereof e.g. esters, then Z may be e.g. an amino function.
The most preferred compounds are acridinium compounds having formula 1 wherein Z is a carboxyl group or a reactive derivative thereof.
The compounds according to the invention can be prepared by per se known methods. The starting material may be acridine or an acridine suitably substituted at position 9, e.g. acridine-9-carboxylic acid, or a derivative thereof, 9-halomethyl-, 9-alkoymethyl-, or 9-aryloxymethylacridine. The acridine-9-carboxylic acid may then be transformed into a suitable ester or acylated sulfon-amide or into another derivative capable of forming a dioxetane after reaction with hydrogen peroxide. This 9-substituted acridine compound may then be quaternized at the acridine nitrogen atom, whereafter spacer A may be built, and consequently a unit carrying group Z may be incorporated. More specifically, the acridine nitrogen atom may be quaternized with the group A-Z wherein the acridine is reacted with a compound Y'-A-Z, wherein Y' is a reactive group e.g. a halogen atom, or another leaving group. It may be profitable to protect the functional group Z during the synthesis to avoid unwanted reactions. Protection of group Z may be performed in a known way: If Z is a carboxyl group this group may be protected in the form of an ester e.g. tert.butyl, benzyl, succinimidyl ester etc.
The acridinium compounds according to the invention show chemiluminescence after reaction with hydrogen peroxide according to the scheme shown in FIG. 2.
The group X is represented in FIG. 2 as the group CR.sub.2 L wherein the groups R are each e.g. hydrogen or alkyl, or together form an oxo group, and L is a leaving group e.g. a halogen atom, an aryloxy group, a sulfonamido group or an ammonium group etc. Upon reaction with hydrogen peroxide and base acridinium compound 1' is transformed into the spirodioxetane 4 with the elimination of the group L. This dioxetane or dioxetanone loses a molecule of CR.sub.2 .dbd.O and produces the excited state acridone with formula 5 which returns to the ground state with light emission. The chemiluminescence can be measured with known instruments, e.g. the LUMAC Biocounter 2010.
The compounds represented by formula 1 show chemiluminescence with a wavelength of generally 420 nm after reaction with hydrogen peroxide. The compound wherein the group X (=CR.sub.2 L) equals a phenoxycarbonyl group and the group A-Z equals a carboxymethyl group, shows chemiluminescence with a wavelength of 420 nm after reaction with hydrogen peroxide. The detection limit of this compound is lower than 8 fg (=8.10.sup.-15 g) corresponding with about 16 attomol (=16.10.sup.-18 mol). As is known the chemiluminescence can have analytical applications. If the compound 1 is coupled as a label to a substrate, and this coupling product is separated from non-coupled acridinium compound, then the intensity of the chemiluminescence upon reaction with hydrogen peroxide is a measure of the concentration of the coupling product, which can be related to the quantity of substrate.
The invention also relates to the use of acridinium compounds represented by formula 1 in immunoassays by means of chemiluminescence.
Herein the chemiluminogenic compound 1 is coupled to a compound that is involved in an immunological reaction, e.g. an antigen. This labelled antigen yields a labelled complex after complexing or other interaction with a compound that is also involved in an immunological reaction, e.g. an antibody. This labelled complex can (after separation of non-complexed antigen) be determined chemiluminometrically in a qualitative, and quantitative way.
The invention also relates to the use of acridinium compounds represented by formula 1 in immunoassays by way of chemiluminescence and energy transfer.
Herein the antibody is coupled to an energy acceptor e.g. a fluorogenic compound, whereafter the antigen-antibody complex can be determined by chemiluminescence and energy transfer without the necessity of separating the non-complexed antigen. The compounds according to the invention are extremely suitable for this kind of assay method because the energy transfer from the acridinium part to an acceptor is very efficient, and because there is no donor (the acridine compound)-acceptor dissociation during the chemiluminescence reaction.
The invention further relates to compounds represented by formula 1, coupled to compounds involved in immunological reactions.
The invention also relates to the use of the above-mentioned coupling products of the acridinium compounds represented by formula 1 with substances which are involved in immunological reactions, for determining immune reactions and immune compounds by means of chemiluminescence whether or not in combination with energy transfer.